Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. Such a dispersant is necessary to produce a colloidally stable mixture and an ink that can be "jetted" reliably without clogging the print head nozzles.
Dispersing agents in an ink jet ink have the dual function of helping to break down pigments to sub-micron size during the milling process and of keeping the colloidal dispersion stable and free from flocculation for a long period of time. In general, dispersions suffer from poor colloidal stability due to particle agglomeration and/or sedimentation, thus limiting the usefulness of the dispersions in inks.
Although a wide variety of dispersing agents are known for pigmented ink jet inks, they are not without certain problems. For example, many dispersing agents are very selective as far as being able to disperse pigments to sub-micron size. In many instances, each class of pigments may require a specific dispersing agent. Another problem encountered with some polymeric dispersing agents is that they tend to impart an undesirably high viscosity to the resulting inks. Thus, there is a continuing need for improved dispersing agents for pigmented inks, especially for non-aqueous inks.
Dispersing agents for non-aqueous ink jet inks previously used include metal salts of styrene-acrylic copolymers, metal salts of sulfonated styrene-acrylic copolymers, phosphonium salts or quaternary ammonium salts of styrene-acrylic copolymers.
U.S. Pat. No. 5,679,138 relates to a process for preparing an ink jet ink wherein the pigment particle size distribution obtained is quite small. There is a problem with this process, however, in that the carrier medium is limited to water. It would be desirable to provide ink jet inks which employ non-aqueous carriers in order to avoid paper deformation upon printing. However, dispersing agents commonly used for aqueous dispersions generally do not work in non-aqueous dispersions.
U.S. Pat. Nos. 5,837,046; 5,739,833 and 5,538,548 relate to ink jet inks containing various Disperbyk.RTM. dispersants used in a variety of carriers such as aliphatic hydrocarbons and dibasic esters. However, there is a problem with these dispersants in that they do not provide very low particle size dispersions when used with alcohol carriers.
It is an object of this invention to provide a method for making a non-aqueous, pigment dispersion for an ink jet ink with a low particle size in order to obtain better covering power and which would have less tendency to clog the ink jet nozzles. It is another object of this invention to provide a method for making a non-aqueous, pigment dispersion for an ink jet ink which employs a dispersant which enables the production of nanometer-size pigment particles.